1. Field of the Invention
This invention relates to the field of pressure sensing gauges and methods of manufacturing said gauges.
2. Description of Prior Art
There are a variety of sensors used to measure pressures in a vacuum in the range of 10.sup.-3 torr to atmospheric pressure (760 torr). Currently the prior art reveals four general types of gauges to measure these pressures. These gauges are thermocouples, ionization gauges, oscillating circuits, and finally pirani gauges. Previous disclosures indicate that there are certain disadvantages associated with the use of each of these various measurement devices.
The use of a thermocouple involves using an electrical current to heat a portion of the device and measuring the resulting temperature of the heated portion by monitoring fluctuations in the electrical voltage of a thermocouple element. As the pressure falls, the rate of cooling of the heated portion by the ambient gas decreases. As a result, either the temperature of the heated portion rises or the electrical current needed to keep the heated portion at constant temperature decreases. The disadvantage of the thermocouple measuring system is the necessity that the heating element and measuring element are separate elements and require separate electrical connections. This increases the size and complexity of this pressure measuring system.
An oscillating circuit usually consists of a quartz resonator and a heating source. The resonator undergoes oscillation depending on its temperature. As the gas pressure decreases, the heat reaching the resonator from the heating source decreases. This results in the resonator changing oscillation. Again, this approach to pressure measurement involves a variety of separate devices and connections.
The ionization gauge involves a heated filament that emits electrons which ionizes gas molecules. The ions are then captured in an ion collector. As the gas pressure decreases, the amount of gas molecules available to be ionized decreases. This ultimately results in less ions to be received by the ion collector. The ionization gauge is large and also requires several devices and electrical connections and is primarily effective only at low pressures (&lt;10.sup.-3 torr) as they will burn out at atmospheric pressures.
The final method is the pirani gauge which is similar to the thermocouple except that the heating element and temperature element are combined into a single wire, thus eliminating multiple devices and electrical connections. In a pirani gauge, the wire is heated and the resistance of the wire is monitored. As the pressure decreases, less heat is transferred from the wire to the surrounding gas. This results in an increased filament temperature which increases the resistivity of the wire. While a pirani gauge eliminates the multiple elements and associate wiring, there are drawbacks to existing pirani vacuum gauges. Specifically, most pirani gauges are large in size, usually much greater than 100 microns. Additionally, pirani gauges are very difficult to manufacture because of the thin wires and connections which need to be made. Finally, pirani gauges measure the resistance change in a metal wire as its temperature changes. Metals have a very small positive temperature coefficient of resistance (TCR). That is, their resistance increases a little bit with an increase in temperature. This coupled with their low resistance makes conventional pirani gauges difficult to measure and relatively insensitive.
There have been attempts to improve pirani pressure gauge technology, but these have met with limited success. A. W. van Herwaarden and P. M. Sarro described an "Integrated thermal vacuum sensor with extended range" in Vacuum, volume 38, number 6, pages 449 to 453, 1988. This device was created using an etching procedure which was also used in the manufacture of integrated-circuit technology. However, the size of the sensor is still very large.
Another attempt to improve pirani vacuum gauges was discussed in "Micro-Pirani vacuum gauge" in the February 1994 issue of Review of Scientific Instruments. Ping Kuo Weng and Jin-Shown Shie fabricated a pirani sensor by silicon micromachining techniques. However, the size of this device is still large and is difficult to manufacture. Weng and Shie use expensive platinum films as their sensing element; these films have a typical resistance of 450 ohms and a TCR of 0.25%/.degree. C. A further disadvantage of the Weng and Shie gauge is the complex processing involved. The platinum must be passivated and the silicon etched with hydrazine solution at 100.degree. C. Furthermore, the process involves at least four or five masking steps.
Deifenderfer (in Principles of Electronic Instrumentation, 2nd ed., Saunders College Publishing Philadelphia, 1978, p. 64-65) describes the operation of a pirani vacuum gauge. His description claims their accuracy is only "2-3 percent, and the resistance changes are seldom more that 10 percent." The resistance of pirani gages are typically in the ohms or tens of ohms so that multiple wire (e.g. Wheatstone Bridge) type measurements are absolutely necessary. For higher accuracy, measurements of the temperature compensation is also necessary, typically utilizing a "dummy" gauge wire.